1. Field of the Invention
This invention relates to automatic power transmissions providing a plurality of gear reduction ratios, such as automatic mechanical transmissions, and to control systems therefor. In particular, the present invention relates to control systems for vehicles with throttle controlled engines and automatic transmissions wherein gear selection and shift decisions are made and executed based upon measured and/or calculated parameters such as output shaft or vehicle speed, input shaft or engine speed, throttle position, calculated engine speed in a potentially engageable ratio, and the like. More particularly, the present invention relates to automatic transmission control systems of the type described above having a selectable mode of operation in which downshifting by more than a single ratio will be commanded if allowable.
2. Description of the Prior Art
The use of automatic transmissions of both the automatic mechanical type utilizing positive clutches and of the planetary gear type utilizing frictional clutches is well known in the prior art as are control systems therefor. Electronic control systems for automatic transmissions wherein gear selection and shift decisions are made based upon certain measured and/or calculated parameters such as vehicle speed, engine speed, rate of change of vehicle speed, rate of change of engine speed, throttle position, rate of change of throttle position, full depression of the throttle (i.e. "kickdown"), actuation of the braking mechanism, currently engaged gear ratio, and the like are known in the prior art. Examples of such automatic transmission control systems for vehicles may be seen by reference to U.S. Pat. Nos. 4,361,060; 4,073,203; 4,253,348; 4,038,889; 4,226,295; 3,776,048; 4,208,929; 4,039,061; 3,974,720 and 3,942,393, all hereby incorporated by reference.
While the above referenced automatic transmission control systems, and similar systems, are effective to control an automatic transmission by selecting a desired gear ratio which will tend to optimize the fuel economy and/or performance of the vehicle in view of the sensed parameters and then to command a shift into the selected gear ratio, such control systems were not totally acceptable as the predetermined programs utilized to generate a shift pattern or patterns (also commonly referred to as a shift point profile) were not modifiable by the operator to select a mode of operation optimizing vehicle performance when ascending a steep grade (as would normally be encountered in off-road operating conditions). In particular, the relatively large inertia of a heavy vehicle ascending a relatively steep grade (for example, a grade of more than twenty percent (20%)) would tend to cause the vehicle to decelerate quickly and result in multiple rapid single downshifts which may be objectionable and cause further vehicle deceleration and/or stalling of the engine due to lack of power supplied to the vehicle from the engine during the out of gear portion of the shift transient.